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While helping some friends remodel their old
house, I re-framed a door opening to fit a wider door. I framed the
header above the door at 82 inches above the finished floor, the
standard dimension for today's 80 inch tall doors. But when I
installed the door, it would barely open because the bottom of the
door would hit the floor. D'oh! I forgot to account for the
upward sloping floor when I framed-in the rough opening.

So I removed the hinge pins and took the
door back to my garage so I could cut an inch off the bottom.

While the door shown here was a
solid wood raised-panel door, trimming the end of a
hollow-core door is similar... as long as you don't cut off
too much.

I placed the
door slab on a suitable work surface... the back
of my utility trailer, which easily fits
in my garage.

I used a couple of old rugs to
protect the door from scratches.

I measured carefully from the bottom of the door
(one inch in my case), then I used a
straight-edge and a mechanical pencil to mark
the cut line.

The straight-edge shown here is
half of an 8-foot long cutting guide, which I
will use later.

I used a
sharp knife to score the wood along the
cut line.

Scoring the wood helps reduce
splintering along the cut caused by the
circular saw teeth pulling the wood fibers
upward.

By The Way: Only the top
surface needs to be scored... the bottom surface doesn't get
splintered when cutting with a circular saw.

I set the circular saw on the edge of the door
and adjusted the cutting depth so the blade
would penetrate the door by about 1/4 inch (red
arrow).

(If I left the blade at full depth,
the motor housing on the saw will hit the
cutting guide.)

Blade Depth Makes A
Difference:

I've noticed a lot of people.
even professional carpenters, who simply leave the
circular saw set to maximum depth for everything they
cut. Over the years I've learned that I get better
control when the blade depth is set so it's just
penetrating the material. By keeping the blade depth to
a minimum, it's easier to turn the saw slightly
if the blade wanders away from the cut line.

Sometimes the circular saw gets lifted up slightly
while cutting, and if the blade doesn't extend far
enough below the material it might not cut completely
through, so I adjust the blade about 1/4 inch deeper
than the material.

I measured
the distance from the left edge of the saw blade
to the left edge of the saw foot.

In my case,
this was about 5-1/8 inches.

Then I clamped the cutting guide 5-1/8
inches away from the cut line.

If a cutting
guide is not available, I will use a good
straight piece of thin wood, such as 1x4 or 1x6.
The factory edge of a sheet of plywood also
works well, but the material must be thin or the
motor housing on the circular saw will hid the
guide.

I placed the
circular saw against the door to see if the
cutting guide was adjusted properly.

The edge
of the saw teeth (red arrow) was just slightly
farther from the end of the door than I wanted,
so I adjusted the position of the gutting guide
by lightly tapping on it with a hammer.

It's important to do the step shown
above to verify that the blade will cut exactly where
needed. I find it's easy to get all these dimensions turned
around in my head and have the blade cut on the wrong side
of the line.

Finally...

After all these careful adjustments, I made
the cut across the bottom of the door, keeping
the saw pressed up against the cutting guide.

Since the
door was a solid wood rail-and-stile type of
construction, the off-cut broke apart at the
finger joints between sections, which was not a
problem.

One advantage of using solid wood
doors is the ability to trim them down to size.
Hollow-core doors have only a narrow
strip of solid wood around the perimeter, and if these
doors are trimmed too much the structural part
can be removed completely, leaving two thin layers of
plywood or particle board with no backing.

Replacing Lost Structure In
Hollow-Core Doors:

Replacing the solid wood
perimeter is simply a matter of fastening an
appropriate-sized piece of wood between the outer layers
of the door. There are 3 issues:

Type of wood: The wood should have straight grain
and be free of knots. Using any old piece of 2x4 framing
lumber isn't the best idea, though you might find a
knot-free straight section of wood within a piece of
framing lumber.

Size:
Cutting the wood to the proper thickness is best
done on a table saw. Standard interior doors are 1-3/8
inches thick, and the facing material is typically about
1/8 inch thick, so the perimeter structure will be approximately 1-1/8 inch thick.

Fastening: The best way to fasten the structure
is to glue it to the outer facing layers with wood glue
or urethane construction adhesive and clamp the assembly
for a day until the glue is completely dry. I've also
driven small brad nails or one-inch staples through the
outer facing (to eliminate waiting for the glue to dry),
but this leaves holes that need to be filled.

After cutting off the end of the door, I ran a
belt sander over the edges of the cut to
break the sharp edge.

A close-up
view of the slight bevel created with the
belt sander.

This procedure can also be done
with a sanding block and sandpaper.

Next, I fastened the hinge leaves to
the door and placed it in the door opening to check the fit.
When the door was open, the bottom of the door just barely
cleared the floor.